An example of a conventional communication method that uses multiple antennas is the communication method known as MIMO (Multiple-Input Multiple-Output). In multi-antenna communication represented by MIMO a plurality of streams of transmit data are individually modulated, and each modulated signal is transmitted simultaneously from a different antenna, thereby increasing the data communication speed.
In this kind of communication method, the receiving side must separate and demodulate a plurality of modulated signals multiplexed together on the transmission path. Therefore, in communication using multiple antennas, demodulation precision has a major influence on the effective data transmission speed.
An example of a heretofore known technology that improves modulated signal reception quality on the receiving side and improves the effective data transmission speed when performing multi-antenna communication is a technique whereby space-time block codes are transmitted as described in “Space-Time Block Codes from Orthogonal Design” IEEE Transactions on Information Theory, pp. 1456-1467, vol. 45, no. 5, July 1999.
These space-time block codes will be explained briefly using FIG. 1. As shown in FIG. 1B, a transmitting apparatus has antennas 5 and 6, and transmits a signal simultaneously from each of antennas 5 and 6. A receiving apparatus receives the simultaneously transmitted plurality of signals with an antenna 7.
FIG. 1A shows the frame configuration of signals transmitted from antennas 5 and 6. A transmit signal A is transmitted from antenna 5, and at the same time, a transmit signal B is transmitted from antenna 6. Transmit signal A and transmit signal B are composed of symbol blocks in which the same symbols are placed a plurality of times so that coding gain and diversity gain are obtained.
This will now be explained in more detail. In FIG. 1A, S1 and S2 indicate different symbols, and complex conjugates are indicated by “*”. In space-time block coding, at time i, symbol S1 is transmitted from first antenna 5 and symbol -S2* is simultaneously transmitted from second antenna 6, and then at time i+1, symbol S2 is transmitted from first antenna 5 and symbol S1* is simultaneously transmitted from second antenna 6.
At antenna 7 of the receiving apparatus, a signal is received in which transmit signal A subjected to channel fluctuation h1(t) between antenna 5 and antenna 7, and transmit signal B subjected to channel fluctuation h2(t) between antenna 6 and antenna 7, are combined.
The receiving apparatus estimates channel fluctuations h1(t) and h2(t), separates the original transmit signal A and transmit signal B from the composite received signal by using those estimates, and then demodulates the symbols.
If space-time block coded signals are used at this time, when signal separation is carried out maximal-ratio combining of symbols S1 and S2 can be performed regardless of channel fluctuations h1(t) and h2(t), enabling large coding gain and diversity gain to be obtained. As a result, reception quality—that is, error rate performances can be improved.
As a different modulated signal is transmitted from each transmitting antenna in multi-antenna communication as described above, it is ideally possible for the data transmission speed to be increased by a factor equal to the number of antennas compared with a case where a modulated signal is transmitted from a single antenna. However, if the precision of separation and demodulation of the modulated signals is poor, the effective data transmission speed will decrease.
In contrast, when space-time block coding technology is used, the reception quality (error rate performances) of the modulated signal transmitted from each antenna can be improved, enabling a decrease in data transmission speed due to degradation of separation and demodulation precision to be suppressed.
However, a drawback with the use of space-time block coding technology is that, while reception quality (error rate performances) certainly improves, transmission efficiency decreases. That is to say, since S1* and −2* transmitted using space-time block coding are demodulated as S1 and S2 by the receiving apparatus, the same information is transmitted twice at time i and time i+1, and transmission efficiency decreases proportionally.
For example, in a general multi-antenna communication system, symbols S3 and S4 differing from symbols S1 and S2 are transmitted at time i+1, and therefore four symbols S1 through S4 can be transmitted in the period from time i to time i+1. That is to say, considered in simple terms, data transmission efficiency when using space-time block coding technology decreases to half that of general multi-antenna communication.